1. Field of the Invention
The present invention is related to a content-addressable memory and more particularly to a content-addressable memory such as an address array in the cache memory realized in VLSI and a translation lookaside buffer for address translation.
2. Description of the Prior Art
As an example of conventional content-addressable memory that facilitates testing, a device introduced in a publication of the 16th Fault-Tolerant computer symposium (1986) of IEEE, page 34 to 39, may be cited.
Such a conventional content-addressable memory consists of a storage bit cells; a comparison means (made up of an Exclusive OR circuit) which has the output of the storage bit cell applied to one of its inputs (first input) and a search input to the other input (second input); and an OR circuit which has the output of the comparison means applied to one of its inputs (first input) and a mask signal of a mask register applied to the other input (second input).
Detecting a fault in the content-addressable memory involves entering a prescribed data into the storage bit cell of the content-addressable memory, supplying the same data as the prescribed data to the second input of the comparison means as a search input, and applying no mask to the output of the comparison means. By checking the non-masked output of the comparison means, it is possible to decide whether there is any fault with the content-addressable memory.
When the content-addressable memory has a wide bit width, say, 32 bits, the data used for memory fault detection becomes large. To avoid this, the mask operation using the mask register and the OR circuit is effective in checking the content-addressable memory. With masking, a partial and detailed fault detection can be made on the content-addressable memory by using only a small amount of fault detecting data.
As mentioned above, with the conventional content-addressable memory, it is possible to eliminate the associative search at any bit location by appropriately setting the mask register. It is also possible to read the content of the result of search (bit register).